# Weird broadcasting behaviour

**URL:** <https://discourse.julialang.org/t/weird-broadcasting-behaviour/39762>\
**Category:** General Usage\
**Tags:** question, broadcasting\
**Created:** [May 19, 2020, 3:54pm UTC](https://discourse.julialang.org/t/weird-broadcasting-behaviour/39762 "2020-05-19T15:54:37Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Henri\_Laurie](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henri_laurie/32/5079_2.png) [@Henri\_Laurie](https://discourse.julialang.org/u/Henri_Laurie)\
**Post date:** [May 19, 2020, 3:54pm UTC](https://discourse.julialang.org/t/weird-broadcasting-behaviour/39762/1 "2020-05-19T15:54:37Z")

</div>

Broadcasting works for matrices up to 10 by 10, then fails:

```julia
julia> n=10; A = rand(n,n); B = ones(n,n);

julia> y .= A .+ B;

julia> n=11; A = rand(n,n); B = ones(n,n);

julia> y .= A .+ B;
ERROR: DimensionMismatch("array could not be broadcast to match destination")
Stacktrace:
 [1] check_broadcast_shape at ./broadcast.jl:509 [inlined]
 [2] check_broadcast_axes at ./broadcast.jl:512 [inlined]
 [3] check_broadcast_axes at ./broadcast.jl:515 [inlined]
 [4] instantiate at ./broadcast.jl:259 [inlined]
 [5] materialize!(::Array{Float64,2}, ::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{2},Nothing,typeof(+),Tuple{Array{Float64,2},Array{Float64,2}}}) at ./broadcast.jl:823
 [6] top-level scope at REPL[265]:1

```

I have no idea why this might be, please help!

---

<div class="post-metadata">

**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [May 19, 2020, 3:56pm UTC](https://discourse.julialang.org/t/weird-broadcasting-behaviour/39762/2 "2020-05-19T15:56:14Z")

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This isn’t about size. The problem is y is 10x10, so you can’t copy an 11x11 array into it